1. Field of the Invention
Zoom lenses which have relatively large zoom ratios typically consist of four lens groups, namely, first to fourth lens groups from the object end. Some zoom lenses of this kind are focused by moving the first lens group and are continuously changed in focal length for zooming by moving the second lens group. The third lens group, which is called a compensator, moves relatively to the second lens group so as to correct a change of focus of the zoom lens which is caused by the movement of the second lens group during zooming.
Since the first lens group is generally larger in diameter and heavier in weight than other three lens groups, moving the first lens group of the zoom lens for focusing exerts a great deal of load on a focusing mechanism of the zoom lens. If the zoom lens incorporates an electrically operated automatic focusing device, the first lens group is too heavy to be quickly and smoothly moved by the electrically operated automatic focusing device. For this reason, some zoom lenses of this kind have a second lens group movable for zooming and a fourth lens group movable for correcting a change of focus during zooming. Such a zoom lens is known from, for instance, U.S. Pat. No. 4,256,381, entitled "Zoom Lens," issued Mar. 17, 1981.
On the other hand, some zoom lenses of this kind have second and third lens groups which move relatively to each other for zooming while first and fourth lens groups remain stationary. The fourth lens group consists of two lens elements or a single lens element.
2. Description of Related Art
In a zoom lens having a fourth lens group moving for focusing, focusing the zoom lens on a subject in a range of short subject distances from the zoom lens causes the fourth lens group to travel a long distance. Accordingly, the zoom lens is not only still hard to quickly focus if operated by an electrically operated automatic focusing device but also is unavoidably long in overall length.
A zoom lens utilizing a stationary fourth lens group, consisting of a single lens element, is short in overall length. However, such a zoom lens is insufficient in its aberration correctability, so as to make it difficult to provide an increase in lens speed and zooming ratio. On the other hand, the zoom lens utilizing a stationary fourth lens group consisting of two lens elements is difficult to be made short in overall length.